1. Field of the Invention
The present invention relates to a stabilized aqueous solution of hydrogen peroxide, and more particularly, to an aqueous solution of hydrogen peroxide which comprises a small amount of a metal element, such as tantalum, zirconium, and niobium, and has excellent storage stability and heat stability.
2. Description of the Related Arts
Hydrogen peroxide is an inorganic industrial material useful as an oxidizing agent. Hydrogen peroxide has widely been used for bleaching fiber and pulp, as germicides, in organic synthetic reactions, and in recent years, for cleaning wafers in the electronic industry. Various stabilizers are generally added to an aqueous solution of hydrogen peroxide to prevent decrease in the concentration of hydrogen peroxide during transportation and storage before the solution is used for the above applications.
The concentration of hydrogen peroxide in an aqueous solution of hydrogen peroxide is decreased mainly because hydrogen peroxide is catalytically decomposed by various metal impurities contained in the solution.
To prevent decomposition of hydrogen peroxide by masking (or sealing) these metals which catalytically decompose hydrogen peroxide, inorganic salts, such as pyrophosphates and stannates, or organic compounds, such as organic chelating agents and organic acids, are added to the solution. Introductory description on decomposition of hydrogen peroxide, elements and compounds showing the catalytic activity for decomposition of hydrogen peroxide, and methods for stabilizing an aqueous solution of hydrogen peroxide can be found in "Hydrogen Peroxide" by SCHUMB et al., published by REINHOLD Publishing Company, New York (1955), pages 447 to 539.
Typical examples of the widely used inorganic stabilizer include phosphates, pyrophosphates, and stannates. These stabilizers are used in large amounts because of low prices. Among these stabilizers, phosphates must be added to an aqueous solution of hydrogen peroxide in large amounts because the single use of a phosphate shows only a limited effect to stabilize hydrogen peroxide although degradation of phosphates by hydrogen peroxide is small.
Solutions for treatment of wafers in the electronic industry, food additives, and germicides are the major areas of application of hydrogen peroxide in recent years. Decrease in the amount of a stabilizer is required in all these applications. When a solution containing a large amount of a stabilizer is vaporized after use, the amount of a residue is increased, and the use of a stabilizer in a large amount is not preferable.
Pyrophosphates which are considered to be effective for masking metal impurities have a drawback in that pyrophosphates are decomposed into orthophosphoric acid in an aqueous solution of hydrogen peroxide at high temperatures to decrease the effect of masking metal impurities to the level shown by phosphates.
Stannates have a drawback in that excessive elusion of aluminum takes place when stannates are stored in a vessel made of aluminum, and the eluted aluminum causes precipitation of stannates themselves. Therefore, stannates cannot be used for practical applications.
As the organic chelate compounds which are considered to be effective, for example, chelate compounds derived from phosphonic acid, such as ethylenediaminetetramethylene(phosphonic acid), ethylenediaminetetraacetic acid, and nitrilotriacetic acid, are described in Japanese Patent Publication Showa 50(1975)-36838. Among these chelate compounds, ethylenediaminetetraacetic acid and nitrilotriacetic acid have a drawback in that these compounds containing nitrogen are decomposed with hydrogen peroxide in a long period of time to cause decrease in the stabilizing effect although these compounds show an effective stabilizing effect in the initial period of the use and are not suitable for using for a long period of time.
Chelate compounds derived from phosphonic acid markedly corrode materials of tanks and vessels during storage to cause elution of metal impurities having activity for decomposition of hydrogen peroxide although these chelate compounds show strong ability to prevent decomposition of hydrogen peroxide, Therefore, a large amount of a corrosion inhibitor for metals, such as sodium nitrate and ammonium nitrate, must be used in combination. This causes the same drawback as that described above in that the amount of a residue after vaporization is increased, and the solution can be used only for limited applications. Moreover, these organic chelate compounds have another drawback in that these compounds are expensive, and the production cost is increased.
As the other organic compounds used as the stabilizer, many organic compounds, such as organic hydroxyl compounds, diglycolic acids, organic sulfonic acids, acylation products of phosphorous acid, phenanthroline, aminotriazines, and acetoanilide, are known. However, these organic compounds have drawbacks in that the single use of these compounds shows a small effect of stabilization, and that these compounds are degraded by oxidation with hydrogen peroxide and the effect of stabilization is lost. Therefore, these compounds cannot practically be used particularly when the solution must be stored for a long period of time or has a high concentration of hydrogen peroxide.